On Feb 17, 2010, at 4:00 AM, Ralf B wrote:
Hi,
I am a beginner in R and have only read a few chapters in the R book,
Which "R book"?
I was not able to find a solution for this simple problem.
I have an empty data frame:
a=data.frame(name="test")
No, you have a dataframe with one column of type "character" and one
row:
> a
name
1 test
which I would like to extend in a for-loop (with data extracted from a
database). Ideally I would like to extend the data frame like this:
a["new_1"] = 1:10
a["new_1"] = 1:12
a["new_1"] = 1:14
You cannot use indexing to "extend" dataframes. Dataframes are not
appropriate for arbitrarily long data objects. Lists are better for
that purpose.
R now obviously complains about the changing length of the new
columns. However, I would like to have missing values being added
whenever columns are shorter than a newer (longer) column.
You can wish for anything, but this is not the default behavior of
dataframes. Under normal circumstances adding a shorter vector will
result in "argument recycling", so to circumvent that you will need to
add the appropriate length of NAs to the end of your vectors:
dtfrm$new <- c(vec, rep(NA, length(vec)- nrow(dtfrm))
# can't use length dtfrm since that is number of columns
# is this like telomeres on chromosomes?
Add adding longer vector than the number of rows with "$ <- "
assignment will cause an error;
You could pre-dimension your dataframe so it is ready to hold your data:
needed_rows <- 1000
> dtfrm <- data.frame(dummy=rep(NA, needed_rows) )
> str(dtfrm)
'data.frame': 1000 obs. of 1 variable:
$ dummy: logi NA NA NA NA NA NA ...
If you then need to add complete rows of new data, you can also use
rbind. You probably ought to look at the mechanisms for accessing
databases directly. Possible search terms: sqlite, sqldf, RODBC
How can I
do that?
Read the help pages:
?"["
?rbind
?cbind # may be the same
Thanks,
Ralf
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
______________________________________________
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and provide commented, minimal, self-contained, reproducible code.
